Gemmological and Spetral Characteristics of Natural and Treated Yellow Akoya Pearl

In recent years, highly saturated yellow Akoya pearls fill the market demand for golden pearls that less than 8 mm, and the sales volume increases rapidly. However, little researches have been done on the gemmological and spectroscopic characteristics of such pearls. In this paper, the gemmological characteristics, UV-Vis spectrum and photoluminescence spectrum of 75 pieces of Akoya pearls of light-yellow to golden colour were obtained. The result shows (1) The natural coloured yellow Akoya pearls has a delicate surface texture and almost no fluorescence under long-wave ultraviolet excitation. Some colour-treated samples show obvious iridescence circle at surface blemish. The uneven fluorescence in yellow or strong bluish white indicates the reagents penetrated. (2) UV-Vis spectrum shows that natural yellow Akoya pearls may exhibit 360-420 nm absorption broad band and 430-470 nm narrow band, and some natural pearl samples with yellow to orange colour show narrow absorption band at 405 nm generated by pigment that have similar structure to uroporphyrin. Colour-treated samples show broader absorption band at violet-blue range, and the strong absorption band below 400 nm may be caused by optical brighten agents. (3) PL spectrum shows that the ratio between fluorescence background intensity and the main aragonite peak (F/A) of naturally coloured samples clustered around 1. Most of the colour treated samples have F/A value above 10. Optical brightening agent treated pearl showed fluorescence maximum at 575 nm and 610 nm.The dyed pearl might show fluorescence information between 630-650 nm. The optical brighteners and dyestuffs might bond and increase the plane rigidity of luminophore, and the intensity of 575 nm might significantly increase to annihilate the dyestuff fluorescence information between 630-650 nm.